The invention relates to a thread cutting device for cutting a needle thread and a bobbin thread after the sewing operation (including zigzag stitching) has been ended with a sewing machine.
Known as a conventional example is a thread cutting device that is disclosed in Unexamined Japanese Utility Model Publication No. Hei. 3-30947 published in Japan before the application of the present invention and that is shown in FIGS. 2(1) to 2(3), 3A(1) to 3B(5), 6, 8(1) to 8(2), 9.
In these figures, a needle hole 26a formed in a throat plate 26 of a sewing machine is such a slender, elongated hole as to allow a needle N to be located at left and right positions so as to swing. The thread cutting device is fixed below a slide 32 on one side in the longitudinal direction of the needle hole 26a.
This thread cutting device includes a movable knife 21, a fixed knife 22, a stopper 23, and a spring 24.
The movable knife 1 has a blade portion 21a formed on the upper surface close to the point thereof, and a thread pickup portion 21b for picking up sewing threads Y formed by having the middle portion thereof indented, and can shuttle between a standby position (FIG. 2(1) and FIGS. 3A(1) and 3B(5)) and an advance position (FIG. 2(3) and FIG. 3A(2)).
The fixed knife 22 has a blade portion 22a arranged on the front end thereof, and has a stepped portion 22b arranged on one side edge of the base end portion thereof. The fixed knife 22 can shuttle between a stop position (FIG. 2(1) and FIG. 3A(1)) and an operating position (FIG. 2(3) and FIGS. 3A(2) to 3A(4)). The blade portion 22a of the fixed knife 22 has an acutely angled blade point as shown in FIG. 6.
The stopper 23 has the base end thereof supported so as to be rotatable. The stopper 23 has a stopper portion 23a arranged on the front end thereof. The stopper portion 23a not only positions and fixes the fixed knife 22 at the operating position while engaged with the stepped portion 22b, but also blocks the fixed knife 22 from returning to the stop position when the fixed knife 22 is moving to the operating position thereof. The spring 24 gives the stopper 23 elasticity in the rotating direction so that the stopper 23 can perform the positioning and fixing operation.
An elongated hole 22c is formed in the fixed knife 22. The elongated hole 22c extends in the moving direction of the fixed knife 22. A projecting portion 25 engageable with the elongated hole 22c is formed on the base end of the movable knife 21. The movable knife 21 is coupled to a drive unit to be described later. In the course of the movable knife 21 making a forward movement (FIG. 2(2)), the projecting portion 25 engages with the right end of the elongated hole 22c to couple the fixed knife 22 to the movable knife 21, and the movable knife 21 therefore continues to make the forward movement as far as to the advance position (the operating position) together with the fixed knife 22. Further, the movable knife 21 has a cam portion 21c formed, the cam portion 21c engaging with the stopper 23 and allowing the stopper 23 to move away from the stepped portion 22b of the fixed knife 22 when the movable knife 21 is making a backward movement.
An operation of this thread cutting device will be described. In a zigzag sewing machine used in the following description, the thread cutting position (the operating position) with the movable knife 21 and the fixed knife 22 is located at the center in the longitudinal direction of the needle hole 26a shown by the one dot chain lines in FIG. 3 independently of needle swinging positions. When a solenoid 27 shown in FIG. 9 has been energized, a drive lever 28 is rotated, so that one end of the drive lever 28 engages with a thread cutting cam 30 fixed to a main shaft 29 of the sewing machine. As the cam surface of the thread cutting cam 30 displaces in accordance with the rotation of the main shaft 29, the drive lever 28 rotates counterclockwise (FIG. 9). An operation lever 32 rotates counterclockwise as viewed in FIG. 9 while pulled by a wire 31 fixed to the other end of the drive lever 29. As a result, a coupling lever 33 coupled to the front end of the operating lever 32 rotates, which in turn causes the movable knife 21 coupled to the coupling lever 33 to make a forward movement from the standby position.
In the course of the movable knife 21 making a forward movement, the fixed knife 22 also makes a forward movement with the projecting portion 25 of the movable knife 21 having engaged with the right end of the elongated hole 22c of the fixed knife 22 as shown in FIG. 2(2). In the course of the fixed knife 22 making the forward movement, the front end of the movable knife 21 advances into a loop Y1a of the needle thread Y1 that is picked up by a beak of a rotary hook portion (not shown) (FIG. 3A(1)).
When the movable knife 21 is in the advance position (FIG. 2(3), FIG. 3A(2)), the needle thread Y1 extending to the needle N and the bobbin thread Y2 between a piece of work W and the rotary hook portion (not shown) are positioned within the indented portion of the movable knife 21 and caught by the front end of the fixed knife 22. Further, at this position, the stopper 23 locks the fixed knife 22 while engaged with the stepped portion 22b of the fixed knife 22.
When the movable knife 21 makes a backward movement, the pickup portion 21b picks up the needle thread Y1 within the indented portion and the bobbin thread Y2 (FIG. 3A(3)). When the blade portion 21a of the movable knife 21 has returned to a position where the blade portion 22a of the fixed knife 22 is located (FIG. 3B(4)), the blade portions 21a, 22a of the movable knife 21 and the fixed knife 22 coincide with each other. As a result, the needle thread Y1 and the bobbin thread Y2 are cut.
As the movable knife 21 thereafter continues to return, not only the cam portion 21c of the movable knife 21 releases the stopper 21 from the stepped portion 22b while pushing the stopper 21 away, but also the projecting portion 25 of the movable knife 21 engages with the left end of the elongated hole 22c of the fixed knife 22. As a result, the fixed knife 22 is caused to return to the standby position together with the movable knife 21 (FIG. 3B(5)).
In the thus constructed conventional thread cutting device, the projecting portion 25 of the movable knife 21 engages with the right end of the elongated hole 22 of the fixed knife 22 so as to allow the fixed knife 22 to follow the movable knife 21 while the movable knife 21 is making a forward movement, and the movable knife 21 and the fixed knife 22 are brought into pressure contact with each other so that threads can be cut with both knives operating in cooperation with each other. As a result, the fixed knife 22 has sometimes been allowed to follow the movable knife 21 disadvantageously by the pressure applied to both knives before the projecting portion 25 engages with the elongated hole 22c while the movable knife 21 is making an initial forward movement. Consequently, the fixed knife 22 destroys the shape of a needle thread loop Y1a while coming in contact with the needle thread loop Y1a before the movable knife 21 advances into the needle thread loop Y1a. This prevents the movable knife 21 from spreading the needle thread loop while advancing into the loop and therefore prevents the cutting of the threads. Further, not only a needle thread portion extending to the needle, but also a needle thread portion extending to the fabric have been picked up together and cut.
Further, when the fixed knife 22 unexpectedly follows the movable knife 21 as described above, the blade portion 22a of the fixed knife 22 may cut the needle thread Y1 or the bobbin thread Y2 accidentally.
Still further, the blade portion 22a of the fixed knife 22 has, in some cases, cut the needle thread Y1 and the bobbin thread Y2 before the blade portion 22a engages with the blade portion 21a of the movable knife 21 due to the pressure applied to both knives when the pickup portion 21b picks up and pulls the needle thread Y1 and the bobbin thread Y2 while the movable knife 21 is making a backward movement (FIG. 8(2)). In this case, the distance between the cutting position and the pickup portion 21b becomes L2, unlike the distance during the normal thread cutting, which is L1 (FIG. 8(1)). Therefore, the following problems have been addressed. The lengths of the cut needle thread extending to the needle and the cut bobbin thread extending to the fabric become short, and this causes the needle thread to come off from the needle, opens up the finishing seams, and causes initial stitches to skip when a next sewing operation is performed, etc.